Fiber optic gyroscopes are used to accurately sense rotation of an object supporting such a gyroscope. Fiber optic gyroscopes can be made quite small and can be constructed to withstand considerable mechanical shock, temperature change, and other environmental extremes. Due to the absence of moving parts, they can be nearly maintenance free. Furthermore, they can be highly sensitive to very low rotation rates that can be a problem in other kinds of optical gyroscopes.
A typical fiber optic gyroscope includes a coiled optical fiber wound on a core and about the axis around which rotation is to be sensed. The optical fiber provides a closed optical path in which an electromagnetic wave is introduced and split into a pair of waves that propagate in opposite directions and ultimately impinge on a photodetector. During use, a rotation about the sensing axis of the core provides an effective optical path length increase in one rotational direction, and an optical path length decrease in the other rotational direction. The resulting path length difference results in a phase shift between the waves propagating in opposite directions. This result is generally referred to as the Sagnac effect. In a fiber optic gyroscope, the phase shift resulting from the Sagnac effect is used to determine rotation around the axis. Specifically, waves propagating in opposite directions interfere when recombined and impinge upon photodetector, which measure the intensity of the combined wave. The output of the photodetector, which is a measure of the amount of interference, is used to determine the phase difference in the counter-propagating beams, and thus is used to determine rotation around the axis.
In many fiber optic gyroscopes, the traveling electromagnetic waves are modulated by placing an optical phase modulator in the optical path on one or both sides of the coiled optical fiber. This modulation is used to overcome directional ambiguity by introducing a phase shift to the incoming and outgoing waves in the optical fiber. As one example, the phase modulation is achieved by applying a modulating signal across the electrodes of the optical phase modulator. Typically, the modulating signal is a square wave with a period equal to twice the transit time of the light through the coil. The modulating signal causes the photodetector to measure the intensity at two different points in the raised cosine interferogram. The rotation rate and direction can then be determined by the difference in the emitted intensity at the two different measured points.
In order to achieve a high level of performance, the bias modulation frequency should equal the proper frequency of the fiber optic gyroscope sensing coil. The proper frequency is typically the frequency that results in the modulation of one of the counter-propagating waves 180 degrees out of phase with the modulation of the other. The value of the proper frequency can be determined from the length of the optical fiber and the equivalent refractive index thereof. By modulating at the proper frequency, quadrature type errors are nearly eliminated. Quadrature errors are unwanted signals that are synchronous with the desired rate signal but are 90 degrees out of phase with the rate signal. The two most common and significant quadrature errors in an fiber optic gyroscope are due to intensity modulation and second harmonic phase modulation (or any even harmonics) generated by the bias phase modulation. Both of these error mechanisms generate an optical signal at the photodetector, which is 90 degrees out of phase with the desired rotation rate signal. The quadrature error signals go to zero when the bias modulation frequency is adjusted to the proper frequency of the sensing coil. As a consequence it is highly desirable for a high performance fiber optic gyroscope to generate the bias modulation at the proper frequency. Furthermore, it is desirable that the clock used to generate the bias modulation frequency be tunable to account for coil length variance or to implement temperature-dependent proper frequency compensation or a proper frequency servo.
In addition to generating a tunable clock to control the bias modulation, a typical fiber optic gyroscope uses a high frequency clock to sample the photodetector signal. When sampling the photodetector signal, the samples at the beginning of each bias modulation period are usually rejected to eliminate the bias modulation glitch. A bias modulation glitch is generated when the bias modulation frequency does not equal the proper frequency. For the difference in time between the change in bias modulation and the loop transit time, both counter-propagating waves are equally phase shifted producing little interference when recombined. For this time period, a rate insensitive large photodetector signal or glitch is created with a width proportional to the timing difference. Even if the bias modulation exactly equals the proper frequency, a bias modulation glitch can arise from the finite time it takes to modulate from one point of the interferogram to another, caused by the slew limit of the bias modulation drive signal. Anti-aliasing filters, with a frequency typically set to half of the photodetector sampling frequency, are used to prevent high frequency noise from aliasing to the demodulation frequency, causing an increase in noise of the measured rotation rate. These same anti-aliasing filters also spread the bias modulation glitches creating the need to reject the photodetector samples significantly contaminated by the glitch. The larger the fraction of the rejected samples, the larger the measured noise. By sampling the photodetector signal at a higher frequency, the anti-aliasing filter frequency can be set higher, decreasing the amount of glitch spreading. With less glitch spreading, fewer samples need to be rejected, resulting in improved noise performance. Therefore, for best noise performance, a higher photodetector sampling frequency is preferred.
Turning now to FIG. 7, an embodiment of a high speed sampling clock generator Previous timing methods have provided a single tunable high-frequency clock from which both the bias modulation clock and the photodetector sampling clock are derived. These methods have typically relied upon high frequency tunable clock circuits that may not be available in all applications. For example, in applications where radiation hardening is required (e.g. some space applications), the number and type of tunable high frequency circuits that are available for use is severely limited.
Thus, what is needed is a system and method for generating a highly tunable clock for the proper frequency and a high-speed clock for the photodetector sampling converter that do not require unattainable radiation-hardened high speed devices.